Theresa May is due to meet EU leaders at the end of June.
Photograph: Toby Melville/Reuters

Brexiters have hit back at leaked Whitehall advice warning of food and fuel shortages if Britain leaves the EU without a deal, with the Conservative backbencher Jacob Rees Mogg calling it “project fear on speed”.

As Theresa May’s ministers prepared to return to Westminster after the Whitsun recess with the cabinet still deadlocked over customs arrangements, a fresh row broke out over the risks of a no-deal Brexit.

Three scenarios drawn up in Whitehall and obtained by the Sunday Times – the worst of the three referred to as “armageddon” – set out the consequences should Britain walk away from the negotiating table.

“In the second scenario, not even the worst, the port of Dover will collapse on day one. The supermarkets of Cornwall and Scotland will run out of food within a couple of days, and hospitals will run out of medicines within two weeks,” a source told the paper.

A spokesman for the Department for Exiting the European Union dismissed the reports, saying: “These claims are completely false. A significant amount of work and decision-making has gone into our no-deal plans, especially where it relates to ports, and we know that none of this would come to pass.”

And pro-leave ministers hit back, with one describing the reports as “hysterical”.

Rees-Mogg, the chair of the European Research Group, a backbench pro-Brexit group, insisted nothing could prevent Britain from importing whatever goods it needed. “Except in limited fields such as arms sales, an exporting nation, in the absence of sanctions, has no legal mechanism to obstruct trade. Hence the Whitehall document is project fear on speed,” he said.

But other Brexiters acknowledged the risks of leaving without a deal, and blamed the lack of preparation on the highest levels of government.

A senior government source said: “The remain negotiating team hasn’t bothered preparing to implement its policy of leaving the single market/customs union, and now business and MPs are finally waking up to the consequences.

“There must now be an urgent change of mentality by the Treasury, Cabinet Office and No 10 to prepare to be a ‘third country’, if we are to stop the UK drifting into being an EU colony.”

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Another senior Tory inside government said: “This is what you get if you have a group of remain-leaning people running stuff.”

The chancellor, Philip Hammond, told MPs last October that he was reluctant to spend taxpayers’ money on preparing for a no-deal Brexit unless absolutely necessary. “I don’t believe we should be in the business of making potentially nugatory expenditure until the very last moment where we need to do so,” he said.

The leaked no-deal scenarios suggested that in the worst case, officials would have to charter planes to airlift medicines into the country, and within a fortnight petrol would also be in short supply.

The government has suggested it would temporarily waive tariffs and border checks on goods entering Britain in the event of a no-deal scenario, in an effort to minimise disruption at borders. But the EU could still halt the flow of goods in the opposite direction.

Meanwhile, EU agreements on everything from medicines regulation to aviation govern key aspects of everyday life, and it has not yet been agreed whether and how Britain could continue to benefit from them as a “third country”.

The papers appear to have been drawn up for the inter-ministerial group on preparedness, which is meant to coordinate the government’s Brexit plans.

Early in the negotiations, May and her senior ministers repeatedly claimed that “no deal is better than a bad deal”, but they have become increasingly alarmed about the risks of crashing out without trade and regulatory arrangements in place.

However, with the cabinet still unable to agree a collective position on what customs arrangements Britain should seek with the EU27 after Brexit, hardline Brexiters on the Conservative backbenches are becoming increasingly bellicose.

An editorial in the Sun newspaper at the weekend claimed a “whiff of panic” was emanating from Downing Street, and said that if May could not “lead us fully out of the EU, with no deal if necessary, she must make way for someone who can.”

May set up two working groups to refine two rival customs plans and report back to her Brexit inner cabinet. But more than three weeks later, the trade secretary, Liam Fox, has acknowledged that his group, which is looking at the prime minister’s preferred new customs partnership model, has met only once.

May must meet her fellow EU leaders at the end of June, and there is continued disquiet among members of the bloc about the government’s lack of progress on setting out its solution to the problem of avoiding a hard border in Ireland.